1. Field of the Invention
The present invention relates to pulsed laser diodes, and more particularly to current drivers for laser diodes having fast current pulse risetime.
2. Description of the Related Art
A laser diode operated in a pulsed mode should be driven by a current driver capable of achieving a fast current pulse risetime with good efficiency and small size. However, semiconductor laser diodes, particularly high power laser diodes formed from an array of interconnected individual laser diode devices, typically have an extremely low on-state resistance which complicates the design of a good pulsed laser driver.
To achieve a particular current waveform into a complex load impedance typically involves one of two fundamental approaches. A current source, such as a high resistive source or a large inductive source, can be used with an opening switch. This approach suffers from two disadvantages, the power dissipation during the interpulse time and the requirement for an opening switch with characteristics of high current, high voltage, fast switch time and low dissipation. The other approach is a voltage source.
FIG. 1 is a schematic circuit diagram of a conventional pulsed laser driver circuit 100 of the voltage source type, in which a capacitor 114 is used as the energy storage element. While the voltage source type circuit 100 has the advantages of simplicity and small size, it has the disadvantage of a long L/R time constant which compromises its ability to produce as fast a current pulse risetime as might be desired. The pulsed laser driver circuit 100 is operated at a voltage VSUPPLY applied at terminal 110, and includes a charging resistor 112, the capacitor 114, a closing switch 132, a laser diode load 142, and a ground 150. The switch 132 typically is a device such as a MOSFET or thyristor. The circuit also includes various inductances and equivalent series resistances (“ESR”) associated with its components and physical layout, which are encircled in phantom lines in FIG. 1 to indicate that they are not intended components of the pulse driver circuit 100. Resistance 118 associated with the capacitor is the principal unintended resistance in the circuit 100. The principal undesirable inductances in the circuit 100 are a load inductance 140, a parasitic inductance 116 of the capacitor 114, a parasitic inductance 130 of the switch 132, and stray inductances 120 generally associated with the circuit assembly. These inductances establish the finite circuit inductance, which is responsible for the long L/R time constant in view of the extremely low resistance load imposed by the laser diode.
The problem with a long L/R time constant, in somewhat simplified terms, is the following. The magnitude of a flat top current pulse may be determined by the source voltage divided by the total circuit resistance. To achieve a given magnitude of flat top current in a low resistance load, a sufficiently large valued capacitance is used and the magnitude of the source voltage is fixed. However, the L/R time constant of the circuit limits the current risetime, regardless of the source voltage. Consider, for example, a need to supply a 1 kA flat top current pulse into a 5 mΩ load, which requires a supply voltage of 5 volts. A pulsed laser diode driver such as the circuit 100 typically has a total circuit inductance of about 10 nH, so that the pulse risetime is limited to 10 nH/5 mΩ or 2 μs. Unfortunately, risetimes faster than 2 μs are sometimes desirable.
The simple approach disclosed in U.S. Pat. No. 5,068,862 issued Nov. 26, 1991 to Zimmermann et al. purports to achieve fast switching by supplementing a battery of capacitors with a smaller capacitor connected in parallel. While this technique may achieve some improvement, the amount of the improvement is limited by the many inductances in the circuit that are unaffected by the presence of the small capacitor, such as the switch inductance, the load inductance, and the stray inductances.
Various approaches have been developed in attempting to overcome the inadequacies of the basic pulsed laser diode driver circuit.
In the approach disclosed in U.S. Pat. No. 5,089,727 issued Feb. 18, 1992 to Molitor et al., a fast-acting switch purportedly is realized from a combination of two critical components, a silicon controlled rectifier and a MOSFET, to obtain at the load a pulse of electrical energy having the desired rapid risetime.
In the approach disclosed in U.S. Pat. No. 5,406,572 issued Apr. 11, 1995 to Chung, U.S. Pat. No. 5,418,807 issued May 23, 1995 to Chung, and U.S. Pat. No. 5,444,729 issued May 23, 1995 to Chung, an optically activated three terminal semiconductor switch is used first to initiate current flow from an energy storage capacitor to a high power laser array with a first switch, then to short circuit the high power laser array to terminate the energy flow with a second switch. The energy storage capacitor is either a uniform stripline structure or a non-uniform stripline structure with a very low impedance.
Alternative approaches such as disclosed in United States Patent Application Publication No. 2003/0016711 published Jan. 23, 2003, and United States Patent Application Publication No. 2003/0039280 published Feb. 27, 2003, use inductors and additional switches to improve current pulse risetime.
Unfortunately, these approaches for overcoming the inadequacies of the simple RC-type driver as well as the alternative approaches cause the pulsed laser diode driver to take up more space and/or to be more complex and expensive.